Role of flower-like ultrathin Co₃O₄ nanosheets in water splitting and non-aqueous Li-O₂ batteries

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are both fundamental and essential processes for various energy conversion and storage systems. The kinetics of ORR and OER play a critical role in their energy efficiency and practicality. Here, flower-like ultrathin Co₃O₄ nanosheets synthesized through a facile solvothermal technique were studied as a bifunctional catalyst for both water splitting and non-aqueous Li-O₂ batteries. Due to the novel structure and highly active {110} and {100} exposed facets, which can effectively facilitate mass transfer and enhance catalytic capability, Co₃O₄ nanosheets exhibit better stability and higher ORR/OER activity than Co₃O₄ nanoparticles, Co₃O₄ bulks, Pt/C, and RuO₂ in alkaline solution. More importantly, Li-O₂ batteries with ultrathin Co₃O₄ nanosheets catalyst can enhance the initial discharge capacity from 6400 to 8600 mA h g^-1 and improve the cyclability up to 160 cycles at 500 mA g^-1. Unexpectedly, XRD and UV/Vis techniques suggest that the main product in Co₃O₄ nanosheets based cathodes is LiOH, with resulting LiOH also demonstrating reversible formation/decomposition behavior, rather than Li₂O₂ in pure Super P based cathodes. Further investigation confirms that Co₃O₄ can also catalyze the electrolyte decomposition responsible for the formation of LiOH, and a reaction mechanism was illustrated. This work highlights that the traditional high-efficiency bifunctional catalyst in aqueous media may not be suitable for non-aqueous Li-O₂ batteries, and the effect of catalyst on electrolyte besides the discharge product should also be carefully considered for the design of more stable and practical Li-O₂ systems.